Process of making creep-resistant zinc-base alloys



ABSTRACT OF THE- DISCLOSURE This invention concerns a method for preparing a fabricated zinc-base alloy article from the atomized alloy which is uniquely characterized as creep resistant for long periods, particularly at moderate stresses, even at elevated temperatures. The method comprises providing a molten zinc-base alloy containing one or more additive metals es- .sentially all dissolved therein, atomizing the molten alloy into finely sized droplets, solidifying said droplets into pellets and fabricating the pellets by hot working into a useful shape thereby to prepare an article having an extremely fine dispersion of second phase.

This application is a continuation-in-part of copending US. patent application Ser. No. 528,791 filedFeb. 21,

1966 now abandoned, which is a continuation-in-part of Ser. No. 323,169, filed Nov. 12, 1963, now abandoned.

The single most critical property of zinc is creep resistance, particularly for long periods. Also desirable is a zinc-base alloy which would be extremely plastic above a certain temperature and/or stress and only difficultly deformable at lower stresses. This would facilitate fabrication of the alloy and avoid failure during extended use below these critical propertyvalues, thereby provide .a design utility for zinc heretofore unrealized.

The zinc-base alloys of the present invention and articles derived therefrom unexpectedly approach or provide these desirable characteristics. For example, a pellet extrusion of the zinc-base alloy containing 0.5% by weight of iron when tested in tension usually offers little or no advantage in strength and is often weaker than its ingot or cast counterpart. It may also exhibit higher ductility. Moreover, when tested for creep resistance under, for example, a high stress of ten thousand pounds per square inch-the zinc-base pelleted alloys of the type disclosed herein deform much more readily than nonpelleted or ingot extrusions. It would, therefore, be expected that such zinc-base alloys are not particularly desirable in pelleted form, as to creep resistance and indeed would be undesirable at any stress load.

- Accordingly, it is surprising to discover, as disclosed herein'that certain'zinc-base pelleted alloys offer outstanding creep resistance if the stress applied is moderate, i.e., below a certain critical value. For example, pellet extrusions of the zinc-base alloys containing chromium or containing calcium with respect to amounts in accordance with the present invention are much worse as to creep resistance than their ingot counterparts at a high stress of about ten thousand pounds per square inch, but exhibit surprisingly superior creep resistance when the creep stress is moderate, e.g., about 6K s.i. This superiority is even more pronouncedwhenthecomparative tests United States Patent ice are accomplished at elevated temperatures such as, e.g., 200 F.

A primary object of the present invention is to provide a method of preparing a zinc-base alloy article from solidified pellets of a defined size which is characterized by enhanced strength and superior creep resistance at moderate stresses.

Another object of the invention is to provide a method of preparing a Zinc-base fabricated alloy article from the atomized metal which is characterized by a retained fine dispersion of second phase and long term creep resistance at moderate stresses, and particularly at elevated temperatures.

A further object of the invention is to provide a novel zinc-base alloy article fabricated from atomized pellets characterized in a hot worked form by a retained fine dispersion of second phase in the zinc matrix and long term creep resistance at moderate stresses, particularly at elevated temperatures.

The term zinc as used herein means the pure metal and commercially available primary zinc containing the normal types and concentrations of impurities.

The term moderate stress as used herein means an applied stress in determining creep resistance of from about 3 to about 8 thousand pounds per square inch (K s.i.), and the term high stress means such stresses of from at least about 10K s.i. and higher.

The terms hot worked or hot working means the metal forming techniques of extrusion, forging, rolling, and/or compaction.

In practicing the invention, at least one additive metal selected from the group consisting of the following operable additive metals, in an amount by weight (based on total weight of alloy) within the broad and preferred ranges indicated below, is dissolved in either pure or commercial grade molten zinc.

OPERABLE ADDITIVE METALS Metal Aluminum Antimony Arsenic Barium Calcium.

Broad range Preferred range Magnesium. Manganese The so-prepared zinc-base alloy is then rapidly solidfied, and thereafter fabricated by hot working into shaped articles.

Of the operative additive metals indicated above, titanium, chromium, calcium, iron, manganese, and copper are preferred in the amounts indicated.

Elements which are normally used in zinc-base alloys, e.g., from 0.5 to about 1.0 weight percent copper, may be employed in the present invention to enhance, e.g., ductility.

Further, it has been found that titanium in an amount within the range of from about 0.01 to about 0.5 percent, and preferably from about 0.1 to about 0.5 percent can be employed in combination with one or more of the operable additive metals aforesaid to further enhance articles at higher stress levels. In addition, the additive metals of magnesium or lithium may be employed in combination with the other additive metals in small amounts to similarly (as with titanium) enhance the creep should be of a size capable of at least passing a number 20 mesh screen (US. Standard Sieve Series), preferably a number 100 mesh screen, and then fabricated into a use ful shape by hot working.

One method of fabrication found particularly desirable resistance of the zinc-base alloy articles of the present 5 is by extruding the pelleted alloy. In this process the invention. pellets are normally preheated to a temperature of from Inasmuch as the greatest improvement in creep resistabout 400 F. to about 700 F. and placed in an exance in the present invention is obtained in the alloys trusion container, which usually is at from about 300 F. used therein when the solidified alloy has a uniform fine to about 600 F., and extruded by subjecting the alloy dispersion of second phase particles in the base metal to a suflicient displacement pressure to express the pellets matrix, the alloy is solidified as rapidly as possible by through a die having the desired extrusion apertureQIf atomizing into finely divided droplets, including jet or desired the pellets may be initially compacted prior to disc type atomizing into pellets. If very fine pellets, e.g., extrusion. those passing through a number 100 mesh or finer screen The following examples serve to further illustrate the (US. Standard Sieve Series) are produced, the disperapplication and utility of the present invention but are sion of second phase is even finer and an even greater not intended to limit it thereto. improvement in creep resistance is obtained than in the A number of zinc-base alloys each having the composimore coarse pellets. tion set forth in Table I were prepared in accordance Fabrication of the alloy in pelleted form into useful with the present invention and atomized into pellets using shapes by hot working may be by extruding, compacting, a jet type atomizer at a temperature slightly above the rolling, or forging. Hot working temperatures and exalloy liquidus. Essentially all of the pellets so-produced posure to elevated temperatures should, however, genwere of a size capable of passing a number 2'0 mesh screen. erally be minimized to minimize agglomeration. A batch of pellets of each alloy was then extruded from The articles made by the novel process of the present a 3-inch extrusion container at about 400 to 500 F. invention, in addition to being significantly and surprisand a rate of about 5 feet per minute into a strip having ingly creep resistant at ambient or slightly elevated tema /8 inch by 1% inch cross-section. p j cllarafiterlzedfiy a Structure compflsing. a Test pieces were then prepared from said strips and Inform d1SPer$1011 1n the zlllc-base metal matnX of tested at room temperature for percent elongation (pertremely small particles of second phase, generally an mcent E) (using a limb gauge length), tensile Strength termetalllc compound. Said particles are of a s1ze not (TS), tensile yield Strength -s (at a 02 percent exceeding abount 0.0001 mch, and preferably not exceed- Oflset), and precent creep extension, b l i a about (100005 lnchtinuous load at one end of the test piece-for a partic- In preparing the alloy of the present invention using ular period of time and at a given temperature as indithe techniques and equipment commonly employed in cated in Table I, and recording the resulting percent the zinc art, commercially pure zinc is liquefied at a temcreep extension after various elapsed time periods perature sufficient to essentially dissolve all the selected (Hours). The rate of creep may be calculated as the difadditive metal, or a combination of additive metals, which ference between the figure representing the total percent is admixed therewith either in particulate form or by inof creep and percent creep after a given elapsed time. troducing small ingots or chunks thereof into the molten The results of these tests are recorded in Table 1' below. zinc While the molten mass is stirred to facilitate dissolu- In addition, for comparison purposes and as controls, tion. The so-prepared zinc alloy is then atomized and a portion of some of the alloys were cast into 3 inch rapidly solidified, preferably as aforesaid by jet or'disc diameter billets (Ingot) and tested as described above, type atomizing into pellets, all or a majority of which with the results also presented in Table I below.

TABLE I Tension tests Creep tests 1,000 p.s.i. Percent creep in- Extr. Per- Percent additive temp., cent Temp, Stress, 10 100 1,000 Example metal F. Form E TYS TS F. 1,000p.s.i. V hrs. hrs. hrs. Other (1123.)

7 30 41 200 3 .03 .07 (at 3,239 hrs.). 7 30 41 78 6 .07 22 22 34 200 3 .05 .15 (at 3,239 his). 26 21 36 200 3 .04 .05 .06 .03 (at 3,239 hrs.). 26 21 36 7s 6 .03 .08 .08

(7) 16 19 35 200 3 .01 .01 .02 .04 (at 3,239 hlS.). Control do 16 19 35 78 6 .10 .46 Control".-- 0.005 chromium--- 450 .do 5 9 31 200 3 .20 .55 (8) 0.5 calcium 450 Pellet 3-100 29 20 38 78 6 .12 .13 .14

S me 22 17 32 200 3 .01 .02 .03 .03 (at 3,164 1115.).

22 17 32 78 6 .09 .36 6 25 37 200 3 .81 31 20 34 78 6 .08 .10 .10 .10 (at 1,393 hrs.). 13 21 31 78 6 .04 .03 .22 .27 (at 1,393 hrs.). 24 20 39 7s 6 .07 .15 .38 .42 (at 1,226 his). 21 24 38 78 6 .06 .44 4.00 13 22 78 4 .10 .20 .30 25 32 42 7s 4 .05 .22 .34 11 27 42 78 6 .04 .06 .06 .06 (at 2,566 hrs.); 6 34 43 78 6 .03 .07 .19 .24 (at 1,726 hrs.). C 16 32 49 7s 6 .01 .02 .02 .03 (at 5,780 hIS.).

1.0 6 15 0.0 9 {lg-50 .06 Cr 16 32 49 7s 10 .06 .08 .10 .14 (at 4,270 hrs.)- 16 663 4 1 0 06 Cr 16 32 49 7s 12 .20 (at; 1,510 hrs.). (17) cola {1330 .06 Cr 16 32 49 78 15 .38 .60 .99 1.65 (at 2,350 hrs.).

. [1. Control..." 0.033413-8006 Or 450 Ingot 22 5 42 8 6 .01 .01 .03 .04 (at 4,438 hrs.).

1 Balance of alloy composition being essentially zinc.

3 Extrusion temperature.

. The values in Table I clearly show the significant improvement in the creep resistance of the pelletedalloys of the present invention prepared by the'novel method thereof over those cast, i.e., ingots. More particularly, Table, I shows the extremely. low creep percentages of the present alloys in pellet form over extendedperiods of, e.g., 1000 hours, at preferred low to moderate stress levels (e.g.., 3 to 6 thousand p.s.i.). This indicates the improved design utility of zinc resulting under the present invention with respect to long term creep resistance. ,Also, it should be noted that additions of titanium in combination with other additive metals generally increases the stress level at which good creep resistance is obtained.

In a manner similar to the foregoing, sodium, potassium, aluminum, antimony, arsenic, barium, cobalt, lithium, magnesium, misch metal, nickel, "thorium andzirconiurn, either singly or in various combinations with each other and with titanium, may also be added to 'zinc and rapidly solidified into pellets from molten droplets then fabricated to obtain high strength zinc-base alloy articles.

Example 18 A melt of commercial grade zinc containing about 0.5 percent by weight iron was prepared and poured into a pot with a plurality of small holes in the bottom each about A; inch in diameter. The molten metal escaped through the holes breaking into droplets as itfell of a size from /s to "A inch, whereupon, the droplets were quenched in a water tank below the pot. This procedure was not in accordance with the present invention. The solidified droplets were extruded into inch diameter rod from a inch diameter container at 500 F. The holes in the bottom of the pot were essentially of the same diameter as the orifice of the atomizing nozzle in the preceding examples. Moreover pressure would have had to be applied to the metal in the pot in order to force it through any smaller diameter holes. As a comparison, atomized pellets in accordance with the present invention finer than 100 mesh and also coarser than 100 mesh of the same alloy were extruded under the same conditions.

Samples of each extrusion were tested for percent elongation (percent E), tensile yield strength (TYS), tensile strength (TS) and percent creep at a 6000 pound load stress for the indicated time in hours with the following results.

Additive metal:

I claim:

1. A process for preparing a creep resistant zinc-base alloy article which comprises the steps of: (1) melting a zinc-base alloy consisting essentially of at least one additive metal selected from the group consisting of the following additive metals substantially all dissolved in the molten zinc in an amount by weight within the following corresponding ranges, the balance being essentially zinc:

Range in percent wt.

(2) atomizing said zinc-base alloy into molten droplets of a size, if solidified as pellets, capable of passing a 20 mesh screen; (3) solidifying the so-prepared zincbase alloy atomized droplets into pellets; and (4) fabrieating the solidified pelleted alloy by hot working into a useful shape, thereby providing a high strength creep resistant zinc-base alloy article; said alloy in the article being characterized by a fine dispersion of particles of second phase in a zinc matrix.

2. The process of claim 1 wherein, in step (1), at least one additive metal is selected from the group consisting of the following metals; titanium, chromium, calr cium, iron, manganese, and copper.

3. The process of claim 1 wherein, in step (1), the

Percent Creep at 78 F. at

s.i. ad for Percent Pellets E TYS TS 1 hr. 6 hrs. 46 hrs. 214 hrs.

Waters quenched 7 33 42 0.145 0. 370 1. 486 4. 662 Atomized +100 mesh 18 20 34 0. 100 0. 113 0. 141 0. 148 Atomized -100 mesh 16 17 33 0. 074 0. 075 0. 090 0. 093

additive metal selected is in an amount within the following range for the respective metal:

Additive metal: Range in percent Wt.

Aluminum 2.0l0.0 Antimony 2.0-6.0 Arsenic 1.0-5.0 Barium 0.2-2.0 Calcium 0.1-0.6 Chromium 0.005-0.4 Cobalt 0.1-0.8 Copper 3.0-4.0

4. The process of claim 1 wherein, in step (1), the selected additive metal is chromium.

5. The process of claim 1 wherein, in step (1), the selected additive metal is titanium.

6. The process of claim 1 wherein in step (3), fabricating of the solidified alloy by hot working is by extrusion.

7. The process of claim 1 wherein the alloy in the zinc-base fabricated article is characterized by a second phase particle size not exceeding about 0.0001 inch.

8. The process of claim 1 wherein the alloy in the zinc-base fabricated article is characterized by a second phase particle size not exceeding about 0.00005 inch.

9. The process of claim 1 wherein, when titanium is employed as the additive metal of step (1) in combination with at least one other of said additive metals, titanium is employed in an amount within the range of from about 0.01 to about 0.5 weight percent.

References Cited UNITED STATES PATENTS Finkeldey 75-178 Chisholm et al. 29-420 Leontis et al. 75-214 X Giuliani et al. 75-178 Foerster et a1 75-214 X Foerster 75-214 X Urban et a1 75-178 X L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner U.S. Cl. X.R. 

